In installations in which material such, for example, as photographic film is being unwound from a roll into a coating station, tension in the material usually is controlled by a pneumatic brake, or in some instances a more sophisticated brake of the eddy current type. More specifically, in such systems material is drawn off the supply roll by a pair of rolls which feed the material toward the process station. In response to changes in tension in the film, a dancer roll moves to actuate the brake to restore the tension to the desired value.
Systems of the type described above work well when the roll diameter is relatively big, since at a large roll radius a given tension in the material results in a fairly high torque at the roll shaft. A problem arises when the material has been wound down to a point at which the core is being approached. At this point, the effective radius of a roll may only be about an inch-and-a-half. Under this condition, if the system is running with a tension of about 5 pounds total, the torque at the roll shaft is only about 71/2 inch pounds. At this value of torque, not only is the brake out of its control range because the torque is too low for the pressure to adjust and control the tension, but also, just the friction in the system usually is higher than that torque level. As a result without any braking at all on the machine the tension is increasing above the desired level.
In the prior art, attempts to solve the problem outlined above have been brute force methods. That is to say, a torque motor or the like is coupled to the unwinding spindle and by means of switching it supplies torque to the spindle to overcome the residual friction. In effect, the entire system is biased to such a level that the brake always is operating and it never sees less than a 10-inch pound torque, for example, at the roll shaft. While such a system is feasible, where a turret is being used to support a plurality of rolls a great deal of hardware and much sequencing of the drive motor is required. This additional equipment is used only for a few minutes during the entire unwinding operation when you are down near the core.
As an alternative to the system described above, a DC motor can be coupled to the unwinding shaft so as to act as a generator and provide braking action during the major part of the unwinding operation. When the unwinding operation has proceeded to a point at which the core is being approached, the DC motor automatically responds to a dancer roll so as to become a motor and drive to supply torque to the system. While this is a solution to the problem, it is an extremely expensive solution.